In a default project, the first 3 audio tracks are stereo tracks. When you add a clip with mono audio, it goes to the first mono track and in the case there is not yet a mono track, it will be added. Normally then it goes to track 4. Remember there are three kind of audio tracks, stereo, mono and 5.1.

Really doesn't matter which tracks your audio is on. You can right-click the "Audio 1" area (left side of Sequence) and use the Delete option, set to "Delete all Empty tracks" and this will clean things up for you if you wish.